Active fixation leads, such as screw-in type leads, typically employ a helical anchor that is extendable from the distal tip of the lead and can be screwed into cardiac tissue to affix the lead distal tip to the cardiac tissue. In addition to serving as an anchor, a helical anchor may also act as a tip electrode for pacing and/or sensing.
Screw-in type helical anchors are made of a smooth biocompatible wire shaped into the form of a helix. The surface of the helical anchor may have a microscopic coating of titanium nitride, which is used for electrical enhancement, but does not provide a mechanical lock to secure the helical anchor in cardiac tissue.
Since the surface of the helical anchor is relatively smooth, there is no mechanical interlock between the helical anchor surface and cardiac tissue once the helical anchor is fixated into cardiac tissue. The helical anchor is free to rotate out of the cardiac tissue in which the helical anchor is imbedded if external forces are significant enough to cause rotation of the helical anchor. Therefore, if any rotation of a lead body or lead inner coil of an extendable/retractable screw-in type lead occurs during the acute phase post implant (e.g., between zero and one to two weeks), the helical anchor has a high probability of unscrewing from the cardiac tissue. The result of the unscrewing may be lead to dislodgment and loss of capture and sensing capabilities. These results could place the patient in a medical emergency depending on the condition of the patient.
There is a need in the art for a helical anchor configuration that enhances the fixation of the helical anchor to the cardiac tissue, thereby reducing the likelihood of dislodgement, loss of capture and loss of sensing.